Conventional high power X-ray tubes typically comprise an evacuated chamber which holds a cathode filament through which a heating or filament current is passed. A high voltage potential, usually in the order between 100 kV and 200 kV, is applied between the cathode and an anode which is also located within the evacuated chamber. This voltage potential causes a tube current or beam of electrons to flow from the cathode to the anode through the evacuated region in the interior of the evacuated chamber. The electron beam then impinges on a small area or focal spot of the anode with sufficient energy to generate X-rays. The anode is typically made of metals such as tungsten, molybdenum, palladium, silver or copper. When the electrons are reaching the anode target, most of their energy is converted into thermal energy. A small portion of the energy is transformed into X-ray photons which are then radiated from the anode target while forming an X-ray beam.
Today, one of the most important power limiting factor of high power X-ray sources is the melting temperature of their anode material. At the same time, a small focal spot is required for high spatial resolution of the imaging system, which leads to very high energy densities at the focal spot. Unfortunately, most of the power which is applied to such an X-ray source is converted into heat. Conversion efficiency from electron beam power to X-ray power is at maximum between about 1% and 2%, but in many cases even lower. Consequently, the anode of a high power X-ray source carries an extreme heat load, especially within the focus (an area in the range of about a few square millimeters), which would lead to the destruction of the tube if no special measures of heat management are taken. Efficient heat dissipation thus represents one of the greatest challenges faced in the development of current high power X-ray sources. Commonly used thermal management techniques for X-ray anodes include:                using materials that are able to resist very high temperatures,        using materials that are able to store a large amount of heat, as it is difficult to transport the heat out of the vacuum tube,        enlarging the thermally effective focal spot area without enlarging the optical focus by using a small angle of the anode, and        enlarging the thermally effective focal spot area by rotating the anode.        
Except for high power X-ray sources with a large cooling capacity, using X-ray sources with a moving target (e.g. a rotating anode) is very effective. Compared to stationary anodes, X-ray sources of the rotary-anode type offer the advantage of quickly distributing the thermal energy that is generated in the focal spot such that damaging of the anode material (e.g. melting or cracking) is avoided. This permits an increase in power for short scan times which, due to wider detector coverage, went down in modern CT systems from typically 30 seconds to 3 seconds. The higher the velocity of the focal track with respect to the electron beam, the shorter the time during which the electron beam deposits its power into the same small volume of material and thus the lower the resulting peak temperature.
High focal track velocity is accomplished by designing the anode as a rotating disk with a large radius (e.g. 10 cm) and rotating this disk at a high frequency (e.g. at more than 150 Hz). However, as the anode is now rotating in a vacuum, the transfer of thermal energy to the outside of the tube envelope depends largely on radiation, which is not as effective as the liquid cooling used in stationary anodes. Rotating anodes are thus designed for high heat storage capacity and for good radiation exchange between anode and tube envelope. Another difficulty associated with rotary anodes is the operation of a bearing system under vacuum and the protection of this system against the destructive forces of the anode's high temperatures. In the early days of rotary anode X-ray sources, limited heat storage capacity of the anode was the main hindrance to high tube performance. This has changed with the introduction of new technologies. For example, graphite blocks brazed to the anode may be foreseen which dramatically increase heat storage capacity and heat dissipation, liquid anode bearing systems (sliding bearings) may provide heat conductivity to a surrounding cooling oil, and providing rotating envelope tubes allows direct liquid cooling for the backside of the rotary anode.
If X-ray imaging systems are used to depict fast moving objects, high-speed image generation is typically required so as to avoid occurrence of motion artefacts. An example would be a CT scan of the human myocard (cardiac CT): In this case, it would be desirable to perform a full CT scan of the heart with high resolution and high coverage within less than e.g. 100 ms, which means within the time span during a heart cycle while the myocard is at rest. High-speed image generation, however, requires high peak power performance of the respective X-ray source.